This invention relates generally to the variable frequency control of an AC motor-pump and specifically to a cost effective apparatus for dynamically varying the speed of an AC permanent magnet synchronous motor-pump or more generally an AC synchronous motor driving a pump or other driven device.
AC permanent magnet synchronous motor (PMSM) pumps are widely used in small to mid-size decorative water fountain and aquarium applications. The vast majority of these pumps are submersible. In PMSM submersible pumps the permanent magnet rotor is in contact with water and is magnetically coupled to the motor stator which is encapsulated in a potting compound. The rotor is provided with an impeller structure which in general is moveably attached to the rotor. The PMSM pump speed is proportional to the driving frequency and the number of poles of the motor as it is with all AC synchronous motors. In contrast to AC induction motors these synchronous motor-pumps (as with other AC synchronous motor-driven devices) either run at the synchronous speed or stall.
PMSM pumps generally are available in low voltage (normally 12VAC) and high voltage (110VAC/220VAC) versions and in various flow rates from approximately 30 gallons/hour up to approximately 10,000 gallons-per-hour. Pump power consumption for these pumps runs from approximately 4 watts on the low end to approximately 750 watts for high flow rate PMSM pumps.
There are a number of methods that have been used for varying the speed of an AC motor. Before the advent of micro-controllers AC motor speed controls had been designed using analog techniques. These controllers were complex, expensive and had maintenance issues due in part to component drift. Since the advent of micro-controllers, such analog controllers have been virtually supplanted by units that employ various pulse width modulation (PWM) schemes to vary the frequency of the motor. Especially with large motors the aim of these schemes is to drive the motor with a PWM generated waveform closely approximating a sinusoid, since a sinusoidal waveform (as in mains power) minimizes motor vibration and heating of the motor windings. Some state-of-the-art pulse width modulation schemes for 3-phase AC motors employ space vector pulse width modulation. A micro-controller implementation of space vector technology is taught by Ramarathrum (U.S. Pat. No. 6,316,895).
In large-motor applications it is critical to reduce vibration and motor winding overheating to avoid premature failure of the motor. A significant amount of on-going research is directed towards developing optimal PWM schemes directed towards reducing unwanted harmonics which contribute to motor vibration (see, for example Czarkowski, D., at. al., (2002). IEEE Transactions of Circuits and Systems—Fundamental Theory and Applications. 48:4, pp. 465–475.)
While use of such PWM schemes is critical in large motor applications even though posing significant theoretical and computational difficulties, such schemes can be overkill for controlling the much smaller fractional horsepower AC synchronous motor driven pumps such as described above.
A much simpler approach has been used specifically to control small submersible AC PMSM pumps of the type in widespread use in small fountain applications. U.S. Pat. No. 6,717,383 teaches an apparatus for effectively controlling these small pumps. The waveform so generated is essentially a variable-width square wave pulse separated by a variable width-dead time. Use of such a simple waveform—while appropriate and cost effective for very small PMSM pumps such as have been widely used in the indoor fountain market—can potentially produce unacceptable vibration when used to control many of the larger PMSM pumps that are also used in the fountain and aquarium industries.
Thus what is needed is a cost effective apparatus to control low-to-medium power AC synchronous motor pumps that provides more complex waveforms than employed in U.S. Pat. No. 6,717,383 while avoiding the computational drawbacks of using conventional, computationally intensive pulse width modulation techniques to generate the AC waveform. It should be emphasized that such cost effective apparatus is also directly applicable to controlling AC synchronous motors, and in the case of some induction motors may afford finer control than many Triac-based, variable-duty-cycle control implementations.